A DC-DC converter accepts a DC voltage and produces a different DC voltage. A DC-DC voltage converter that provides a DC output voltage that is less than a DC input voltage is sometimes referred to as a “buck” voltage converter. A switched-capacitor voltage converter is a power converter that uses capacitors and switches to perform voltage conversion using a charge transfer method. A multiple gain switched capacitor voltage converter can be used to select from among different gains to use to convert an input voltage provided by a voltage source such as a battery, to an output voltage level required by an output load.
In a battery powered system, a battery's voltage typically drops as the battery is discharged during its operational lifetime. At the beginning of a battery's operational life, the battery voltage may be higher than a desired load voltage level, so that a step-down converter may be used to provide the power supply voltage. Toward the end of the battery's life, the battery voltage may be less than a desired load voltage level, so that a step-up converter may be used to provide the power supply voltage level. A multiple gain switched capacitor voltage converter may be used, for example, when there is a need to vary voltage gain in order to maintain a desired load voltage level despite changes in a battery's voltage level over the course of the battery's operational life.
Often, a battery acts as a voltage source used to power system that includes a variety of components, referred to collectively as a “load”, such as a processor, DRAM and Flash storage, for example. To conserve power, some systems employ a low power mode of operation in which certain components such as DRAM and Flash are shut off and other components such as a processor, for example, remain turned on, but operate in a low power mode having a reduced voltage level and a reduced load current level. The reduced voltage and the load current are lower than voltage and current levels during normal operation mode levels. In the past, DC-DC switched capacitor voltage converters have been used during low power mode to convert a battery voltage level to a low power voltage level, while also minimizing battery ‘leakage’ current during low power mode operation.
FIGS. 1A-1B are illustrative drawings representing configuration of capacitors of a prior switched capacitor voltage converter coupled between a battery voltage and a load circuit during a charge phase (FIG. 1A) and a discharge phase (FIG. 1B). low power mode operation, the converter converts a battery voltage level, VBat, to a low power voltage level applied to the load, e.g., to a system in a low power mode. More specifically, the converter switches between the charge and discharge phases at a prescribed frequency. Referring to FIG. 1A, during the charge phase, sometimes referred to as ‘phase I’, N switched capacitors are coupled between the battery voltage, VBat, and ground. During the charge phase, a low power voltage level and load current IL are applied to the load using a reservoir capacitor CRes, which is connected in parallel with the load, which is disconnected from the N switched capacitors. Referring to FIG. 1B, during the discharge phase, sometimes referred to as phase II, the N switched capacitors are coupled in parallel with each other and in series with CRes. The discharge of the voltages on the N switched capacitors recharges voltage on CRes. Switches RSW typically include FET devices that are used to switch the switched capacitors between phase I and phase II, and that act as resistive elements during phase II.
In the prior converter of FIGS. 1A-1B, different numbers of switched capacitors can be used to achieve different voltage gains. For example, in some earlier converters, use of N=2 switched capacitors during the charge and discharge phases results in a gain of ½; use of N=3 switched capacitors during the charge and discharge phases results in a gain of ⅔; and use of N=4 switched capacitors during the charge and discharge phases results in a gain of ¾. Thus, for example, in some prior converters, when converting using N=2 switched capacitors, the converter does not use one-third of the capacitor area.